With increasing energy prices and the environmental impact associated with traditional energy sources, battery-driven cars, generators, and other products are becoming cheaper and more practical alternatives. To power many of these larger products, batteries must be packaged into multiple-cell battery packs, a design that has special requirements. For example, for a battery pack to be both efficient and long-lasting, its individual cells must be balanced. This is especially true for high-voltage battery packs used in electrical vehicles (EVs), hybrid electrical vehicles (HEVs), and industrial applications.
Many EVs and HEVs use lithium-ion (Li-ion) cell battery packs. These cells have high energy densities, high open-circuit voltages, and low self-discharge rates. They are also relatively light. Unfortunately, multi-cell Li-ion batteries are prone to failure when the charges on the individual cells are not balanced. For example, when the cells are not balanced, charging lower-charged cells to their capacity results in overcharging the higher-charged cells. Such overcharging can be dangerous and is particularly damaging for Li-ion cells, which cannot tolerate over charging. When discharging higher-charged cells, a lower-charged cell may be discharged below an acceptable limit, again resulting in cell damage. Both types of damage occur in EVs and HEVs, which have large numbers of recharging and discharging cycles.
Proper balancing requires that each cell be charged or discharged by an amount that brings all the cells to a similar voltage. This in turn requires accurate cell voltage readings, difficult in EV and HEV applications, where motor vibrations, variations in temperatures within the engine compartment that houses the battery-pack, variations in the measurement components themselves, time lags between measuring the individual cells, all can reduce the accuracy of the voltage readings. When differences between cell readings are inaccurate, cell balancing is also inaccurate, reducing the likelihood that the cell balancing will protect the cells as intended.
In previous multi-cell battery systems having n cells, balancing occurred by sensing a voltage on a cell in one polarity and then reversing the polarity and sensing that voltage. This process continued for each cell. Temporal displacement of these measurements introduced errors into the balancing process.
In another prior balancing technique, the voltages on cells 1 though n were sensed in a first polarity. The polarity was reversed and the process repeated with the other polarity. This process also introduced temporal displacement errors into the balancing process.
Temporal displacement errors can be eliminated by adding a voltage sensor for each cell and simultaneously sensing the voltage for all cells. Unfortunately, this technique is expensive.
Many cells are required to power vehicles, such as EV or HEV vehicles, which use relatively expensive battery packs. Because an entire pack must be replaced when any individual cell in the pack fails, such failures are costly in EVs, HEVs, and other applications that use multiple cells.